Frame synchronization, i.e., the search for an assigned time slot, is the first task that a mobile station performs when it is attempting to synchronize to a given digital channel, either a traffic or a control channel.
FIGS. 1A-1D depict the frame, slot, and slot synchronization (SYNC) word sequences and SYNC word usages, respectively, for an exemplary prior art digital Time Division Multiple Access (TDMA) cellular air interface known in the art as IS-136 (see, for example, IS-136.1, Rev. A, Mar. 21, 1996 and IS-136.2, Rev. A, Feb. 12, 1996).
FIG. 1A shows that a 40 millisecond frame consists of six time slots (other TDMA systems may use more or less than six slots per frame). Slots 1-3 and 4-6 each comprise one TDMA Block. In the forward direction from a base station to a mobile station, which is a case of most interest to this invention, the frames are continuously transmitted. A given mobile station is assigned to receive in one slot per frame for a half data rate case, and is assigned to receive in two time slots for a full data rate case. FIGS. 1Ba and 1Bb illustrate the format of one slot in the direction from the base station to the mobile station (i.e., the forward direction). The base station forms a part of a Base Station/Mobile Switching Center/Interworking function (BMI).
In particular, FIG. 1Ba illustrates one time slot of a Digital Control Channel (DCCH) that is transmitted by the BMI on a forward channel. The 28 bit (14 symbol) SYNC word/time slot identifier field occupies the first 28 bits of the frame, and is followed by various other fields, including a first Shared Channel Feedback (SCF) field, a first part of a DATA field, a Coded Superframe Phase (CSFP) field, a second part of the DATA field, a second SCF field, and a Reserved (RSVD) field of two bits.
FIG. 1Bb illustrates one time slot of a Digital Traffic Channel (DTC) that is transmitted by the BMI on a forward channel. As in the DCCH, the 28 bit (14 symbol) SYNC word/time slot identifier field occupies the first 28 bits of the frame, and is followed by various other fields, including a Slow Associated Control Channel (SACCH) field, a first part of a DATA field, a Coded Digital Verification Color Code (CDVCC) field, a second part of the DATA field, a one bit Reserved (RSVD) field, and a Coded Digital Control Channel Locator (CDL) field.
Of most interest to this invention is the SYNC word field that is sent at the beginning of each of the DCCH and DTC slots.
The SYNC word/time slot identifier field is used for slot synchronization, equalizer training, and time slot identification. The six unique synchronization sequences shown in FIG. 1C are defined so as to have good autocorrelation properties to facilitate synchronization and training. Six time slot identifiers are defined, which have good cross correlation properties. Line 8 of FIG. 1D identifies the SYNC words for a channel fully assigned to full-rate users (i.e., three full-rate users per frame). Line 1 of FIG. 1D identifies the SYNC words for a channel fully assigned to half-rate users (i.e., six half-rate users per frame). Lines 2 through 7 identify the SYNC word order for a mixture of full-rate and half-rate users, such that only one SYNC word is assigned per user. Unassigned slots are indicated by the base station as half-rate user slots in the time slot identifier field. The mobile station uses its assigned SYNC word when transmitting on a Reverse Digital Traffic Channel (RDTC).
When a mobile station first comes to a digital channel it locates and then synchronizes to a DCCH. From the DCCH the mobile station is able to determine various access and other network parameters, and from which the mobile station is eventually handed off to a Digital Traffic Channel (DTC). When switching to a DTC, either in response to an initial DTC assignment or in response to a handover command from one DTC to another DTC, the mobile station must be able to quickly and accurately locate its assigned downlink slot or slots by correlating received SYNC fields with one of the SYNC word sequences (see FIG. 1C) that are stored in the mobile station. Referring also to FIG. 5, and assuming that the mobile station is assigned to slot 1, correlations using the SYNC word sequence assigned to slot 1 that are performed over received slots will produce a correlation peak for received symbols that match the corresponding slot 1 SYNC word sequence. The locations of the correlation peaks is thus an indication of the location of the mobile station's assigned slot 1 within the continuously transmitted forward DTC.
It can be appreciated that a number of frames may be required to be received before a high enough confidence level is obtained as to the location of the mobile station's assigned slot(s), especially when receiving a noisy or fading signal. As such, a considerable delay may be incurred when first acquiring the DTC and/or the possibility exists that a call may be dropped during a handover from a first DTC to a second DTC.
It should be remembered that under conventional practice the mobile station does not know how the received DTC frame is being allocated to other mobile stations. For example, the SYNC word usage for a given frame may be any one of the eight cases shown in FIG. 1D. As such, the mobile station is not enabled to predict which SYNC word sequences will appear in slots four through six, and cannot thus utilize any a priori knowledge of other SYNC sequences in these slots in an attempt to synchronize to its assigned slot or slots.